The role of Ca++ in tubulin polymerization has been further investigated. Ca++ interacts with pure 6S tubulin and systems containing microbubule associated proteins (MAPs) as well. The effect is intrinsic to these preparations but is sensitized by the presence of calmodulin. The latter protein requires the presence of MAPs to inhibit polymerization and current efforts are directed toward identifying which of the associated proteins is the primary regulator.